COAL-TAR 301 



into the ovens from wagons which run on rails over the set of 

 ovens. The gas and other volatile products given off from the 

 coal are conveyed away through pipes to a series of collecting 

 wells and purifiers and a gas-holder. The gas, having then de- 

 posited nearly all the tar and ammoniacal liquor, is brought back 

 to a series of burners where, with admixed air, it is burnt in the 

 spaces between the ovens, the walls of which are thus raised to 

 a bright red heat. The products of combustion, chiefly carbon 

 dioxide and water vapour, are then carried into an underground 

 flue, as shown by the arrow, and so to the chimney-stack. By 

 means of apparatus of this kind great economy is secured. There 

 is a yield of about 70 per cent of coke equal in quality to the 

 ordinary coke, together with all the tar and ammonia which the 

 variety of coal used is capable of yielding. 



In some districts where the manufactures can be combined 

 together the surplus gas from the coke ovens, of a quality which 

 is up to the present-day standard of luminosity, is delivered into 

 the ordinary coal-gas mains. 



For the purpose of generating combustible gas a variety of 

 systems have been adopted by which air in limited quantity is 

 drawn or driven through a mass of red-hot coal contained in a 

 firebrick chamber called a " gas-producer." A gas of this kind 

 contains about one-third of its volume of carbonic oxide mixed 

 with nitrogen from the air and a little carbon dioxide, etc. 

 The ammonia is partly destroyed, but of late years a method 

 introduced by the late Dr. Ludwig Mond has been very exten- 

 sively adopted whereby a cheap coal slack can be used, at the 

 same time that nearly the whole of the nitrogen of the coal can 

 be recovered in the form of ammonia. To secure this result the 

 temperature must not be allowed to rise too high, and the yield 

 of ammonia is augmented by driving into the coal a considerable 

 quantity of steam. A portion of this is decomposed with forma- 

 tion of hydrogen gas and carbonic oxide, but the chief effect is 

 to keep down the temperature. The increased collection of 

 ammonia is attended by a considerable reduction in the value 

 of the accompanying tar, which is quite different in character 

 from the tar obtained at the gas works and from the coke ovens. 

 The latter is produced at a much higher temperature, and its 

 characteristic ingredients are hydrocarbons, which are related 

 more or less closely to benzene (benzol) and its homologues. The 

 tars produced at lower temperatures contain very little of these 



